The present invention relates to electron ray apparatuses and more particularly to a scanning electron microscope where secondary electrons generated from a sample and reflective electrons are detected at high efficiency and images with high resolving power and high contrast are obtained favorably.
A scanning electron microscope is such that electrons emitted from an electron source are accelerated and are focused by an electrostatic lens or a magnetic field type lens to make a narrow beam of electrons (primary electron ray); the primary electron ray is scanned onto a sample to be observed using a scan deflector; a secondary signal generated secondarily from the sample by irradiation of the primary electron ray is detected; intensity of the detected signal is made a brightness modulation input of a cathode ray tube which is scanned in synchronization with the scanning of the primary electron beam; and thereby a two-dimensional scan image can be obtained.
The secondary signal generated from the sample by the irradiation of the primary electron ray has wide energy distribution. For example, the primary electrons impinging on the sample are subjected to elastic scattering by atoms on the surface of the solid and some electrons leave the surface of the sample. These are called reflective electrons and have energy equivalent to the primary electron ray or considerably high energy. Also the primary electrons impinging on the sample mutually act with atoms within the sample, and the electrons within the sample are excited and obtain the kinetic energy so that some electrons are emitted to the outside. These are called secondary electrons and have energy in a range from 0 eV to about 50 eV.
Japanese Patent Laid-open No. Hei 7-192679 discloses a technique that using a deflector for deflecting the secondary signal outward from the axis, orbits for the secondary electron and the reflective electron are separated and detectors are arranged on the orbits respectively whereby the secondary electron and the reflective electron can be detected selectively.
Also Japanese Patent Laid-open No. Hei 9-171791 discloses a technique that a reflecting plate provided with an aperture allowing an electron ray to pass therethrough is installed on upper side of a secondary electron detector, and reflective electrons colliding with the reflecting plate are converted into secondary electrons and detected.
Also Japanese Patent Laid-open No. Hei 8-273569 discloses a technique that divided annular detectors are installed on upper side of an objective lens, and secondary electrons and reflective electrons are separated from each other and detected utilizing the focusing function of the objective lens.
According to the techniques disclosed in these references, the secondary electrons and the reflective electrons having different orbits can be detected individually or in combination due to difference of the energy.
The reflective electron or the secondary electron has inherent information individually due to difference of generating cause. According to the techniques disclosed in the above-mentioned references, the reflective electron or the secondary electron can be detected selectively and thereby the sample image based on the inherent information can be formed.
According to the techniques disclosed in the above-mentioned references, while the secondary electron or the reflective electron can be detected individually or in combination, there are following problems.
In Japanese Patent Laid-open No. Hei 7-192679, the respective detectors or reflecting plates are arranged on the orbits for the secondary electron and the reflective electron, and the secondary electron and the reflective electron are detected selectively. Regarding the detection of the reflective electrons, however, the electrons only reflected substantially along the optical axis of the primary electron ray are detected but the reflective electrons generated from the sample at a low angle can not be detected.
This problem applies also to the reflective electron detection technique by the reflecting plate disclosed in Japanese Patent Laid-open No. Hei 9-171791. Also in this case, the electrons only reflected substantially along the optical axis are detected but the reflective electrons generated from the sample at a low angle can not be detected.
Also according to the technique disclosed in Japanese Patent Laid-open No. Hei 8-273569, secondary electrons are raised by an objective lens (electrostatic objective lens), and orbits for the secondary electron and the reflective electron are separated by the focusing function of an objective lens (magnetic objective lens) to detect the electrons. However, since a detector has a detecting surface in the vertical direction to the optical axis of the electron ray, the detecting surface can not be enlarged. Consequently a problem exists in that it is difficult to detect reflective electrons generated at a low angle, reflective electrons generated at a high angle or secondary electrons in discrimination.
Also in any of the objective lenses disclosed in the three references as above described, a lens gap (gap between magnetic poles) is formed toward the vertical direction to the optical axis of the primary electron ray. As a result, a problem exists in that the distance between the lens main surface and the sample (focal length) becomes long.
In order to solve the above-mentioned problems, an object of the present invention is to provide a scanning electron microscope where reflective electrons generated from a sample at a low angle can be detected without lengthening the focal length.
In order to attain the foregoing object, the present invention provides a scanning electron microscope which obtains a two-dimensional scan image of a sample, comprising: an electron source; a scan deflector for scanning a primary electron ray generated from the electron source onto the sample; an objective lens for forming a focusing magnetic field onto the surface of the sample, said focusing magnetic field focusing the primary electron ray; and a secondary signal detector for detecting a secondary signal generated from the sample by irradiation of the primary electron ray, characterized in that electrodes generating secondary electrons are arranged between the objective lens and the secondary electron detector.
According to the above-mentioned constitution, among secondary signals generated by the irradiation of the primary electron ray, a secondary signal generated from the sample at a low angle can be determined.